1. Field of the Invention
The subject invention pertains to a method of obtaining a spatial mapping of an object utilizing nuclear magnetic resonance (NMR). More particularly, the subject invention relates to a method of differentiating between types of tissue having different relaxation times, such as normal versus ischemic muscle tissue, using NMR. The method is particularly effective where the period during which data can be acquired is brief.
2. Background Art
A variety of techniques for NMR spatial mapping are known in the art. For example, in one technique, resonance absorption is measured in a number of planes, and these data are combined in a manner analogous to that used in computerized axial tomography to yield a three-dimensional representation of the object being studied.
In another technique, resonance at a single point is studied by combining three magnetic fields each of which has a linear gradient and a superimposed sinusoidal time dependent oscillation. The time dependent factor is normalized so that there is no time dependent component to any of the three fields at the point being studied. The sinusoidal variation superimposed on the magnetic field outside of the region being studied will cause the resonance effects of such areas to average out over time. Once the magnetization has been established (assumed to be in the Z direction), a short RF pulse is applied in the X direction which causes the magnetization of the sample to precess into the Y direction (in a rotating reference frame). When the RF field is turned off, the magnetization will precess around the Z axis (in the laboratory frame).
In this technique, a series of 90.degree. RF pulses about the X axis separated by a time constant T.sub.0 are applied. The magnetization perpendicular to the static magnetic field will decay at a rate determined by the relaxation time T.sub.2, and the magnetization along the Z axis will decay at a rate determined by the relaxation time T.sub.1. These alternating pulses continue for a period of time, and eventually a steady state is reached. The magnetization in the XY plane can be detected, and this provides a measure of the presence of protons (i.e., water) in the sample being studied. It should be noted that until the steady state condition is approached, only half the cycles contribute to the acquisition of useful signals. Because of the time necessary to reach the steady state, this technique is not particularly useful for analyses which must be done in a short period of time.
In a variation of this technique, two linear time dependent magnetic field gradients are set up (instead of three), and accordingly the resonance condition is satisfied along a line instead of at a point. A linear time independent gradient field is imposed along the line to be studied. In this manner, the precession frequency along the line being mapped is a function of the field strength along the line. A Fourier analysis of the precessional frequencies provides information as to the relative amplitude of the resonance occurring along the line. Thus, the amount of water present can be measured along the line. However, because this method utilizes a series of 90 degree pulses, the even Fourier components do not provide useful data. Thus, this method provides less than optimal resolution. The apparatus used to carry out the foregoing methods is conventional in design.
It has been found that both the relaxation time T.sub.1 and the relaxation time T.sub.2 are about 10% longer in ischemic tissue than in normal tissue. However, because the above-described technique yields a result which is proportional to the ratio of T.sub.2 to T.sub.1 plus T.sub.2, ischemic tissue cannot be distinguished. Furthermore, because the above-described technique generally takes on the order of T.sub.1 to reach steady state operation, it is not suited to study of tissue such as in vivo heart muscle which is only stationary for on the order of T.sub.2 /5 seconds.